This invention relates generally to a manually operated fluid pump dispenser having a hollow piston mounted on a hollow stem for relative sliding movement during piston reciprocation within the pump cylinder to open and close the discharge through the stem. More particularly, the present invention provides for the retraction of product into the pump chamber at the commencement of each piston return stroke to avoid the formation of any dribbles or drips of product at the discharge opening. The product retraction feature is effected by a modification of the pump cylinder without the need for any additional parts, thereby producing significant savings in assembly operation and capital costs involved in producing the dispenser.
As described in the aforementioned related application, the annular piston having a central bore is mounted on the hollow stem of the pump plunger for reciprocation within the pump cylinder which defines together with the pump piston a variable volume pump chamber. A valve controlled inlet passage leads into the chamber, and a valve controlled discharge passage defined by the hollow stem and terminating in a discharge exit opening, leads away from the pump chamber. The piston is mounted on the stem for relative sliding movement during pumping to open and close the discharge port leading to the discharge passage.
The friction force acting between the piston and the inner wall of the pump cylinder is typically greater than the friction force existing between the bore wall of the annular piston and the confronting outer wall of the plunger stem. Thus, during each pressure stroke as the operator depresses the plunger against the force of a return spring, the plunger stem, by reason of such difference in friction forces, shifts in advance of the piston a given distance as determined by engaging stops acting between the piston and the stem.
At the commencement of the piston return stroke, because of the higher friction force acting between the piston and the cylinder versus the friction force acting between the piston and the stem, the plunger stem shifts upwardly under the action of the biasing force of the return spring in advance of movement of the piston to thereby close the discharge port as the piston engages a valve element on the lower end of the stem, thus raising the piston during the return stroke together with the plunger and its stem. The shifting piston during its return stroke thus expands the volume of the pump chamber which reduces the internal pressure below atmospheric and induces product to be drawn from the inlet passage via an unseated check valve and into the pump chamber to reprime the pump. During the pump chamber repriming operation, the discharge valve remains closed while the inlet valve is open.
An object of the present invention is to improve upon the manual pump dispenser of the type aforedescribed as having a relatively shifting annular piston which during pressure and suction strokes opens and closes a discharge port leading to a discharge opening through the stem. In accordance with the invention, a product retraction feature has been developed by a unique modification of the pump bore to effect product retraction into the pump chamber to avoid the formation of any droplets of product at the discharge exit opening. The provision of such a feature requires no additional parts, but simply a restructuring of an existing part thereby avoiding any increase in overall cost of the dispenser. Adaptation of the pump dispenser in accordance with the invention for product retraction is a simple and straightforward procedure yet highly effective and economical.
Pursuant to the invention, the annular piston at the beginning of each piston return stroke initially shifts outwardly of the pump bore together with the plunger stem thereby maintaining the discharge port open such that the discharge passage remains in communication with the now expanding pump chamber causing a pressure drop therein. This reduction in pressure while the discharge remains open a short interval retracts product from the discharge passage into the expanding pump chamber to thereby avoid the unsightly and unwanted bubble/dribble formation of product at the exit end of the discharge passage.
According to the invention, the interference fit between the piston and the pump cylinder wall is adjusted at the lower end of the cylinder for decreasing the friction force acting therebetween below that of the friction force existing between the wall of the central bore of the annular piston and the surrounded plunger stem. This enables the stem to shift the piston together therewith at the commencement of the plunger return stroke thereby maintaining the discharge in an open position during a short interval of the piston return. As the discharge passage remains in open communication with the expanding pump chamber, the negative pressure created by the expanding chamber pulls product from the discharge passage into the chamber while at the same time starts to pull product into the chamber through the inlet passage. The effect is a withdrawal of product inwardly away from the exit end of the discharge path.
In accordance with another embodiment of the invention, the inlet valve has a restoring spring means biased in the direction of reciprocation and lying in the path of reciprocation of the piston. Thus at the end of each pressure stroke, the piston impacts against the spring means causing the piston to shift together with the plunger stem during the ensuing return stroke thus retaining the discharge open and in communication with the expanding pump chamber during a short interval. The negative pressure thus created by the expanding pump chamber functions to draw product into the pump chamber from the discharge passage and to draw product into the pump chamber through the inlet passage. The product is thus retracted away from the terminal end of the discharge to avoid the formation of any dribbles or drips thereat.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompany drawings.